A potential method has heretofore been employed for detecting defects such as cracks were initiated in the metal materials. The potential drop method has been described, for example, in Japanese Patent Publication No. 2593/1975 and Japanese Patent Laid-Open No. 160054/1982.
The conventional apparatus for detecting defects on the surfaces of materials based upon the potential drop method employs a so-called four-terminal method. According to the four-terminal method which employs a probe in which are arranged in alignment a pair of power supplying electrodes and a pair of potential difference-measuring electrodes on the inside thereof, the surfaces of metallic structural members are scanned to measure the change in the potential distribution thereby to detect surface defects such as cracks. That is, the potential difference in a flawless region is used as a reference potential, and it is determined that a defect exists in a place if the potential difference there is greater than the reference potential.
In a large metal structure, initiation of surface cracks does not mean that its life has expired; i.e., the structure in many cases can be used for considerably extended periods of time even after defects have been initiated. The life after the surface defects have been initiated may often be longer than the period before the defects are initiated. It is therefore important to monitor the progress of surface defects from the standpoint of using the structure effectively for extended periods of time maintaining safety.
According to the conventional apparatus having a pair of power supplying electrodes, however, the electric current is not uniformly distributed due to a change in the shape of a member in a measuring region and makes it difficult to determine the reference potential for measurement. Furthermore, determination of the shapes of surface defects lacks precision which makes it difficult to precisely monitor the growth of surface defects. Due to the spread of current, furthermore, inspection is affected by the change of shapes near the measuring regions and by the neighboring defects.